1. Field of the Invention
This invention relates to stabilization of the amount of soluble iron salts in an autodeposition bath using continuous flow filtration.
2. Statement of the Related Art
In the autodeposition coating of iron-containing (e.g. steel) surfaces using an aqueous bath comprising a latex, optionally a pigment, and an activator system itself comprising at least hydrofluoric acid and a water-soluble ferric salt, the amount of iron salts in the bath is quite critical. Too small an amount produces no autodeposition. Too large an amount interferes with the coating process, unfavorably altering the coating characteristics of the bath.
Paradoxically, although a certain amount of ferric salts are necessary for the activation of the bath, the autodeposition process itself removes iron from the continually treated workpieces at a rate of about 20-100 mg Fe/ft.sup.2 (about 200-1,000 mg Fe/m.sup.2) of surface treated. In a large-scale continuous commercial application, such losses can translate into an iron build-up in the autodeposition bath to concentrations in excess of 3 g/l. At such concentrations, the bath interferes with the coating process, and the entire bath, including expensive resin components present, has been discarded in favor of a fresh bath.
Autodeposition baths may also contain small amounts of "tramp" (contaminating) salts such as sodium and potassium phosphates and sulfates, which are "dragged-in" from earlier steps of the continuous process.
Thus, there is a recognized need for a means of controlling and stabilizing the amount of iron (ferric) salts in the autodeposition bath at a predetermined amount.
In recent years, some iron ion control was achieved by adding phosphoric acid to autodeposition baths. This resulted in a precipitation of ferric phosphate which subsequently led to massive coprecipitation of latex particles. Non-ferric contaminant salts were unaffected. The end result was a sludge-laden bath that resulted in erratic performance and control characteristics.
U.S. Pat. No. 4,414,350 discloses complexing the iron salts, present in excess concentration in an autodeposition bath, with at least one carboxylic acid such as acetic, succinic, acrylic, and the like. This also resulted in a sludge-laden bath.
More recently, as disclosed in copending U.S. patent application Ser. No. 06/725,470, which is commonly assigned herewith and whose disclosures relating to iron salt build-up are incorporated herein by reference, a method of bath stabilization (i.e. iron ion control) has been proposed in which one continuously or intermittently discards a predetermined volume of the bath, which volume is replaced by an equal volume of water, the discard rate being chosen so as to maintain a substantially constant concentration of iron salts in the bath. Independently of this, the other ingredients of the bath (latex, pigment, acid, etc.) are replenished at a rate sufficient to maintain each at its original concentration. The results of this method have been excellent with regard to bath performance and stability. However, a considerable amount of latex and pigment is irretrievably lost, which increases the cost of the autodeposition process.